Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $20.87   6.641%   $22.256   $22.26   $22.25   $22.30  
  Purchase #2 $19.69   6.229%   $20.9165   $20.92   $20.90   $20.90  
  Purchase #3 $156.28   6.356%   $166.2132   $166.21   $166.20   $166.20  
  Purchase #4 $23.60   8.754%   $25.6659   $25.67   $25.65   $25.70  
  Purchase #5 $154.42   12.424%   $173.6051   $173.61   $173.60   $173.60  
  Purchase #6 $110.69   4.288%   $115.4364   $115.44   $115.45   $115.40  
  Purchase #7 $109.66   7.476%   $117.8582   $117.86   $117.85   $117.90  
  7 purchase totals = $595.21       $641.9513   $641.97   $641.90   $642.00